# Full adder boolean logic question

1. Jan 16, 2004

### david90

Full adder has input A, B and Cin. what is Cin for? Full adder adds 3 bits right?[?] So why not just call the input A,B,C instead of A,B,Cin?

2. Jan 16, 2004

### chroot

Staff Emeritus
A full adder adds two one-bit numbers, A and B. The Cin line is the "carry-in" line, which is asserted when a less-significant bit's full adder overflowed. In other words, in the following problem:

01 + 01

The least significant bit's full adder would be adding 1 + 1, giving an output of zero and a carry of one. That carry would be passed to the next bit to the left, which would add 0 + 0 + the carry bit, producing an output of 1 and a carry of zero.

If you'd like to simply think of the full adder as adding three one-bit numbers, you can, but that's not the clearest way to think of it. There are two carries: Cin, and Cout.

- Warren

3. Jan 16, 2004

### david90

say you have a circuit that contains 4 full adder thus it is a circuit that could add 8 bits. Would the circuit still function the same if the far right full adder doesn't have a Cin? What i'm thinking is that why it needs a Cin when there is no full adder before it with Cout. Not sure if i'm making sense. Thanks for ur respond!

4. Jan 16, 2004

### chroot

Staff Emeritus
A circuit composed of four full-adders can add two four-bit numbers. The rightmost full adder would have its Cin grounded.

- Warren